The present invention relates to the exploration and production of petroleum from earth formations, and more particularly to methods for determining the amount of oil present in such a formation.
In the petroleum industry, one of the most valuable and informative techniques for determining the characteristics of an earth formation located well below the surface, and the nature of the fluids which it may contain, is to remove and bring a portion of the formation to the surface for analysis. This is most commonly done by "coring" the formation. Of course, physical conditions in the formation are substantially different from those at the surface: pressures and temperatures are ordinarily enormously elevated over surface conditions. Therefore, fluids and gases present in porous rock samples very often evolve from those samples as they are recovered from the formation. To the extent that such liquids and gases are lost, the accuracy of the evaluation of the formation production potential is accordingly impaired.
To control this problem, a technique called "pressure coring" is often employed. With pressure coring, the core is contained at substantially its original formation pressure until proper analysis can be made. Pressure coring, while overcoming fluid loss problems to a great extent, is quite expensive.
A recently developed alternative technique, "sponge coring", shows typical savings of around 70 percent of the cost of pressure coring after considering the savings in both the cutting of the core from the formation and the subsequent analysis of the core. In sponge coring, as the core is cut, it enters a typically half-inch thick polyurethane sponge liner inside the inner core barrel. As the core is brought to the surface, the sponge then captures any formation fluids that escape from the core when the pore pressure drops below the oil's bubble point. Such expanding gas bubbles can displace otherwise immobile oil, and any such oil which bleeds from the core (and this can be as much as 50% of the original core fluid) is thus caught and retained by the sponge liner. An analysis is then made of the oil captured by the sponge, and by adding that to the amount left in the core, one can obtain much more accurate oil saturation values for the formation from which the core was removed.
The importance of coring in the production of petroleum has recently been increasing as more and more secondary and tertiary recovery is being made of petroleum reserves. In a formation undergoing primary production, the original reservoir fluids are little altered from their conditions for the last several thousand years. They may migrate as the oil is produced, but their properties are little changed. However, when fluids and/or other compounds are injected into a formation to stimulate its production, the nature of the connate fluids is accordingly altered, sometimes to a very substantial extent. When this occurs, the more traditional wellbore logging tools may be unable to provide further useful information. In all too many instances, the only way to determine how much oil is left, and thus whether it can be produced economically, is to go down to the formation and take a core sample.
It will therefore be appreciated that the analysis of the oil content of the core sample can be critically important. The final true residual oil saturation of a formation is a determination that can make or break a multi-million dollar enhanced recovery project.
As explained in greater detail in the above-referenced '334 application, a major disadvantage of sponge coring has been the inability to accurately measure the amount of oil retained by the sponge. Many techniques for oil determination have been used by service companies, including mechanical extraction, retorting, and solvent extraction. The problems with these techniques, as they are presently practiced, include incomplete extraction of oil, mistaking extracted sponge components for oil, and in the case of solvent extraction, incomplete removal of extracting solvent before measuring oil volume.
The above-referenced '334 application discloses a substantial improvement in determining the amount of oil in a sponge core. Several solvents are identified which have the unique capacity, previously unrecognized in this industry, to remove substantially all of the oil captured by the sponge without affecting the sponge or dissolving (usually unreacted) sponge components. Highly accurate determination of the oil captured in the sponge is thus made possible, thereby effectively realizing the enormous savings potential of sponge core technology.
There does remain a need, however, to be able to practice this substantial improvement in an efficient and commercially practical manner such that substantial quantities of sponge core (often several hundred feet of core in a typical coring operation) can be analyzed quickly and efficiently. For example, when sponge core is manufactured, the sponge is foamed in place inside an aluminum liner. The sponge then bonds fairly well to the aluminum, making removal of the sponge from the liner difficult and time consuming. In analyzing the sponge core, it would therefore be preferably to analyze it in place in the liner barrel.
A need thus remains for a convenient, effective, and comercially practical method and apparatus for removing the captured oil from the sponge, preferably without having to remove the sponge from the aluminum liner. A need also remains for a method for accurately analyzing the actual amount of oil removed when solvents such as the Freon-11 identified in the above-referenced '334 application are used. Typically, a large amount of solvent is used, resulting in a highly diluted solution, since there is usually not much oil present. Removing the solvent in order to improve the accuracy of the determination of the oil is not as easy as simply evaporating the solvent because the light (e.g., C.sub.5 -C.sub.8) hydrocarbons from the captured formation oil will usually also evaporate.